The present invention relates to a class D amplifier (digital amplifier) which converts an analog signal such as a music signal to a pulse signal, and power-amplifies the signal, and more particularly to a circuit technique for driving and controlling output power MOS-transistors.
Conventionally, a class D amplifier is known which receives an analog signal such as a music signal as an input signal, converts the signal to a pulse signal, and then power-amplifies the signal. An output terminal of the amplifier is connected to an input terminal of a loudspeaker via a low-pass filter. In such a class D amplifier, a pulse signal is power-amplified while reflecting the amplitude (information components) of the input signal to the pulse width, and the pulse signal is output. The pulse signal is then passed through the low-pass filter, whereby the power-amplified analog music signal is extracted. The loudspeaker is driven by the music signal. A class D amplifier can be formed on a silicon chip, and hence realized in a small size and in an economical manner, so that it is widely used in a portable terminal device or a personal computer which is requested to consume a small power.
FIG. 7 shows the configuration of a class D amplifier 900, and an application example of the amplifier.
Referring to the figure, a signal source SIG is a source of an analog music signal VIN in which the midpoint of the amplitude is set to the ground potential (0 V), and connected to an input terminal TI of the class D amplifier 900 via an input capacitor (not shown) for cutting off a DC component contained in the music signal. The class D amplifier 900 is a so-called PWM amplifier (PWM: Pulse Width Modulation), and configured by an input stage 901, a modulating circuit 902, a drive controlling circuit 903, and n-type power-MOS transistors 904 and 905.
The input stage 901 moves the midpoint of the music signal VIN to convert the music signal VIN to a signal conforming to the input characteristics of the modulating circuit 902 which is operated by a power supply VDD (for example, 10 V). The modulating circuit 902 converts the music signal output from the input stage 901 to a pulse signal by the PWM modulation so that the music signal is modulated to a pulse signal while information components of the music signal are reflected to the pulse width. On the basis of the pulse signal which is modulated in the modulating circuit 902, the drive controlling circuit 903 complementarily drives and controls the output power-MOS transistors 904 and 905.
The power-MOS transistor 904 in which the current path is connected between a positive power supply VPP+ (for example, +50 V) and an output terminal TO is used for outputting a high level. The power-MOS transistor 905 in which the current path is connected between a negative power supply VPP− (for example, −50 V) and the output terminal TO is used for outputting a low level. The output terminal TO is connected to a loudspeaker SPK via a low-pass filter consisting of an inductor L and a capacitor C.
In the class D amplifier 900, the music signal VIN supplied from the signal source SIG is passed through the input stage 901 and the modulating circuit 902 to be converted to a pulse signal. In the conversion, the modulating circuit 902 performs PWM-modulation on a carrier signal in accordance with the music signal VIN. On the basis of the modulated pulse signal, the drive controlling circuit 903 complementarily controls the conduction states of the power-MOS transistors 904 and 905, and outputs the power-amplified pulse signal to the output terminal TO. In the power-amplified pulse signal, the carrier frequency component is removed away by the low-pass filter consisting of the inductor L and the capacitor C, to be formed as a power-amplified analog music signal. The signal is then supplied to the loudspeaker SPK.
The modulating circuit 902 is configured so as to be operated by the single power supply VDD (for example, 10 V). Consequently, the low level of the pulse signal which is the output signal of the circuit is equal to the ground potential (0 V), and the high level is equal to the voltage (10 V) supplied from the power supply VDD. When the pulse signal having such signal levels is used as it is, the power-MOS transistor 904 in which the drain is connected to a positive power supply VPP+ (for example, +50 V) cannot be sufficiently controlled to the on state because of the characteristics of a MOS transistor, and the power-MOS transistor 905 in which the source is connected to the negative power supply VPP− (for example, −50 V) cannot be sufficiently controlled to the off state. Therefore, the drive controlling circuit 903 must have a function of controlling the power-MOS transistors 904 and 905 on the basis of the pulse signal which is modulated in the modulating circuit 902.
Hereinafter, the drive controlling circuit 903 will be described. In order to control the conduction states of the power-MOS transistors 904 and 905 which output a signal changing between the positive power supply VPP+ and the negative power supply VPP−, a pulse signal of a large amplitude corresponding to the positive power supply VPP+ and the negative power supply VPP− is requested to be supplied from the drive controlling circuit 903 to the gates of the power-MOS transistors 904 and 905. In this case, the drive controlling circuit 903 must be configured by using high-breakdown voltage transistors, thereby causing the production cost to be increased. Therefore, the drive controlling circuit 903 is configured by employing a technique in which effective power supply voltages applied to circuits for respectively driving the power-MOS transistors 904 and 905 are lowered by isolating power supply systems of the circuits from each other.
In the example shown in FIG. 7, both the power-MOS transistors 904 and 905 are of the n-type, and hence the power supply system of the drive controlling circuit 903 is separated into a power supply system based on the source voltage of the power-MOS transistor 904, i.e., the voltage of the output signal appearing at the output terminal TO, and that based on the source voltage of the power-MOS transistor 905, i.e., the voltage supplied from the negative power supply VPP−. The power supply system of the circuit for driving the power-MOS transistor 904 is raised with following the voltage change of the output signal appearing at the output terminal TO. When the power supply system of the drive controlling circuit 903 is configured so as to follow the output signal appearing at the output terminal TO, however, the input threshold of the drive controlling circuit 903 is varied with respect to the level of the pulse signal output from the modulating circuit 902 in the preceding stage, thereby causing a disadvantage that the signal cannot be correctly transmitted from the modulating circuit 902 to the drive controlling circuit 903.
As a first conventional technique which can solve the disadvantage, there is a technique in which the bootstrap circuit technique is used for raising the pulse signal output from the modulating circuit 902 to a signal level conforming to the drive controlling circuit 903.
As a second conventional technique, there is a technique in which an insulating transformer is used for converting the voltage of the pulse signal output from the modulating circuit 902 to a signal level conforming to the drive controlling circuit 903.
As a third conventional technique, there is a technique in which a photocoupler is used for converting the pulse signal output from the modulating circuit 902 to an optical signal and transmitting the optical signal toward the drive controlling circuit 903.
In the first conventional technique, a bootstrap circuit is used in order to convert the level of the signal output from the modulating circuit, and hence there is a problem in that the operation becomes unstable when the signal has a high frequency.
In the second and third conventional techniques, since electronic components such as the insulating transformer and the photocoupler are relatively expensive, the production cost is increased. Moreover, a space for mounting such electronic components must be assured, and hence the whole amplifier is bulky.
In the conventional configuration shown in FIG. 7, the modulating circuit 902 is operated by the power supply VDD of a 10-V system. If all blocks of the input stage 901, the modulating circuit 902, and the drive controlling circuit 903 are operated by the positive power supply VPP+ and the negative power supply VPP− which are high voltage systems, it is not required to convert the signal level, and the circuit configuration can be simplified. In this case, a production technique of a high-breakdown voltage process must be used for all the blocks. Even when the blocks are formed into separate ICs, therefore, the production cost of each IC is increased.